The subject matter disclosed in this specification is related to the subject matter disclosed in the copending, commonly-assigned U.S. patent application (U.S. Ser. No. 09/659,847) filed by the present inventor, entitled xe2x80x9cLASER APPARATUS IN WHICH SURFACE-EMITTING SEMICONDUCTOR EXCITED WITH SEMICONDUCTOR LASER ELEMENT AND HIGH-ORDER OSCILLATION MODES ARE SUPPRESSED,xe2x80x9d filed on Sep. 11, 2000. The contents of this patent application are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a laser apparatus which generates a laser beam by exciting a surface-emitting semiconductor element with a semiconductor laser element.
2. Description of the Related Art
Conventionally, narrow-stripe single-transverse-mode semiconductor laser devices are used as a light source in the fields of high-speed information processing, image processing, communications, laser measurement, medicine, printing, and the like, as well as in image display apparatuses such as laser display apparatuses.
However, the conventional narrow-stripe single-transverse-mode semiconductor laser devices, which emit high quality laser beams, have a drawback that the practical output power is at most about 200 to 300 mW. There are two causes of this drawback. The first cause is a so-called spatial hole burning. That is, since the rate of carrier supply for generating laser light is limited by the carrier diffusion process when the output power is high, carrier density decreases in a region where the laser beam intensity is high. Due to the spatial hole burning, the refractive index of the semiconductor medium increases, and the waveguide mode is affected by the increase in the refractive index. Thus, the quality of the laser beams deteriorates, and kinks are produced in the current-light output characteristics. The second cause of the above drawback is the great optical output density. For example, in the case where the stripe width is 4 micrometers, and the equivalent beam diameter in the direction perpendicular to the junction is 0.5 micrometers, the optical output density reaches 15 MW/cm2 when the output power is 300 mW. Therefore, heavy load is imposed on the semiconductor medium, and various characteristics of the semiconductor laser device deteriorate. Thus, it is difficult to obtain a reliable, practical narrow-stripe single-transverse-mode semiconductor laser device which emits laser light with high output power. In other words, it is difficult to increase the practical output power of the narrow-stripe single-transverse-mode semiconductor laser device.
In order to alleviate the above first cause, attempts have been made to optimize the waveguide structure, In addition, in order to alleviate the above second cause, attempts have been made to optimize protection coatings at end surfaces, and window structures at end surfaces have been developed. However, these techniques are approaching their limits. Therefore, in order to realize a narrow-stripe single-transverse-mode semiconductor laser device which emits laser light with high output power, a new mode control technique is necessary, and the optical density must be reduced by increasing the light emission area.
Various attempts have been made to realize a high-power semiconductor laser device which emits spatially coherent laser light, and has an output power of hundreds of watts or more. For example, Botez and Scifres, xe2x80x9cDiode Laser Arrays,xe2x80x9d Cambridge University Press, 1994 discloses a monolithic structure realizing a high quality laser beam, and a process for producing the structure. However, the structure and process are complicated. In addition, in the above attempts, the driving currents of the high-power semiconductor laser devices are 500 mA, 1 A or greater. Therefore, when such high-power semiconductor laser devices are modulated, the amplitudes of the modulation currents become 500 mA, 1 A or greater. Thus, heavy loads are imposed on the driving circuits, and the cost is increased. Further, the variation of the large currents generate strong electromagnetic waves. Therefore, a special electromagnetic shielding is needed. That is, complicated arrangement is required.
In order to remedy the drawbacks of the conventional current-injection type semiconductor laser devices, U.S. Pat. Nos. 5,461,637 and 5,627,853 propose surface-emitting semiconductor laser devices which are excited with light. However, since these semiconductor laser devices utilize the thermal lens effect, i.e., the effect of increasing refractive indexes with temperature, the temperature must be raised. In addition, the above semiconductor laser devices are sensitive to temperature distribution, and the spatial oscillation mode is unstable. The spatial mode becomes further unstable when output power is high, since a cavity is generated in a carrier distribution due to generation of laser light having high output power (i.e., the spatial hole burning occurs), and refractive indexes decrease with increase in the number of carriers due to the so-called plasma effect.
As described above, in the conventional laser apparatus in which a surface-emitting semiconductor element is excited with a semiconductor laser element, a large current is needed to modulate the semiconductor laser element which excites the surface-emitting semiconductor element. Therefore, it is very difficult to obtain a high-speed-modulated laser beam by direct modulation of the semiconductor laser element which excites the surface-emitting semiconductor element.
The object of the present invention is to provide a high-quality, high-output laser apparatus which can be directly modulated with a small current.
According to the present invention, there is provided a laser apparatus comprising: a semiconductor laser element which emits first laser light having a first wavelength; a surface-emitting semiconductor element which is excited with the first laser light, emits second laser light having a second wavelength which is longer than the first wavelength, and has a first active layer and a first mirror arranged on one side of the first active layer; a second mirror which is arranged outside the surface-emitting semiconductor element so that the first and second mirrors form a resonator in which the second laser light resonates; and a modulation unit which modulates the surface-emitting semiconductor element.
According to the present invention, the surface-emitting semiconductor element is modulated by the modulation unit, while an excitation semiconductor laser element which excites the surface-emitting semiconductor element is modulated in the conventional laser apparatus. Therefore, according to the present invention, high-speed modulation can be achieved with a small modulation current, while a large current is needed for modulating the excitation semiconductor laser element in the conventional laser apparatus.
Preferably, the laser apparatus according to the present invention may also have one or any possible combination of the following additional features (i) to (xii).
(i) When the surface-emitting semiconductor element has a pn junction, the modulation unit can modulate the surface-emitting semiconductor element by varying a voltage applied to the pn junction. The voltage applied to the pn junction is, for example, a reverse bias voltage. When a reverse bias voltage is applied to the pn junction, the laser gain can be decreased. Thus, it is possible to achieve modulation with a high extinction ratio.
(ii) When the surface-emitting semiconductor element has a Schottky junction, the modulation unit can modulate the surface-emitting semiconductor element by varying a voltage applied to the Schottky junction. In this case, modulation of the laser light can be achieved by controlling the gain of the active layer of the surface-emitting semiconductor element.
(iii) The surface-emitting semiconductor element may comprise a structure for controlling a spatial mode of the second laser light. In this case, the surface-emitting semiconductor element can operate in a fundamental transverse mode even when the output power is high.
(iv) In the laser apparatuses having the additional feature (iii), the above structure may be realized by a pinhole spatial filter being arranged at a light-exit end surface of the surface-emitting semiconductor element, having a pinhole, and allowing passage of the second laser light through only the pinhole. In this case, the resonator loss in high-order modes becomes greater than the resonator loss in the fundamental mode. Therefore, the oscillation in the high-order modes can be effectively suppressed, and resultantly oscillation in the fundamental transverse mode is realized.
(v) In the laser apparatuses having the additional feature (iii), the first mirror may have a limited area, be arranged in parallel with a light-exit end surface of the surface-emitting semiconductor element, and realize the above structure. Since the laser light in the fundamental transverse mode can be selectively reflected by the second mirror, the resonator loss in high-order modes becomes greater than the resonator loss in the fundamental mode. Therefore, the oscillation in the high-order modes can be effectively suppressed, and resultantly oscillation in the fundamental transverse mode is realized.
(vi) In the laser apparatuses having the additional feature (iii), the first active layer may be formed in only a limited area in a plane parallel to a light-exit end surface of the surface-emitting semiconductor element, and realize the above structure. In this case, the resonator loss in high-order modes becomes greater than the resonator loss in the fundamental mode. Therefore, the oscillation in the high-order modes can be effectively suppressed, and resultantly oscillation in the fundamental transverse mode is realized.
(vii) In the laser apparatuses having the additional feature (iii), the above structure may have a size which is 0.1 to 10 times as large as a diameter to which the second laser light spreads at the position of the above structure. In this case, the characteristics and the beam shape of the output of the laser apparatus are particularly improved.
(viii) The laser apparatus according to the present invention may further comprise a wavelength selection means arranged in the resonator.
(ix) The laser apparatus according to the present invention may further comprise a polarization control means arranged in the resonator.
(x) The semiconductor laser element may have a second active layer made of an Inv1Ga1-v1N material, and the first active layer of the surface-emitting semiconductor element may be made of an Inv2Ga1-v2N material, where 0 less than v1 less than v2 less than 1.
(xi) The semiconductor laser element may have a second active layer made of an InGaN material, and the first active layer of the surface-emitting semiconductor element may be made of an AlGaInP or GaInP material.
(xii) The semiconductor laser element may have a second active layer made of an Inw1Ga1-w1As material, and the first active layer of the surface-emitting semiconductor element may be made of an Inw2Ga1-w2As material, where 0 less than w1 less than w2 less than 1.
In the above items (x) to (xii), xe2x80x9ca XY materialxe2x80x9d means a material which contains at least the elements X and Y when each of X and Y is a symbol of an element.
Details of the above additional features (iii) to (xii) are disclosed in the aforementioned copending and commonly-assigned U.S. patent application filed by the present inventor.